1. Field of the Invention
This invention relates to optical systems for amphibious cameras housed within waterproof packages usable either in the air or under water.
2. Description of the Prior Art
The application of waterproof construction techniques to cameras to enable shooting not only in the air but also under water has been well-known for many years and has resulted in a wide variety of so-called amphibious cameras. In this kind of camera, focusing of the photographic optical system results in the prevailing problems which are described below:
(1) In the photographic optical system having a distance adjusting ring, since the object distance is estimated with the eye and then the distance adjusting ring is operated by hand in conformance therewith moving the focusing member to bring the image into focus, the degree of image sharpness is very high. In underwater photography, however, because the photographer must perform while holding his breath, and even worse that under water he loses a stable base on which to stand, he has difficulty manipulating the distance adjusting ring correctly and may have focusing problems.
(2) In the fixed-focus photographic optical system, the focusing position is fixed from the beginning. This offers the advantage that there is no need to perform difficult focusing operations, but gives rise to an alternative problem as stated below: That is, although the camera is usable under water, photographic situations which may be generally encountered are often in the air. Therefore, the optical system is designed to have the above-identified fixed focusing position on an object at a distance suited particularly for in-the-air photography. Specifically, for in-the-air photography, it is most convenient to design the photographic optical system in such a way that an object at a distance of about 3 meters in the air is sharply focused at an image plane with a field depth covering an object distance range from about 1.5 meters to infinity.
The conventional photographic optical systems of the kind described above, when put into water shifts the aforesaid focusing position to a farther object distance than when in the air. At this time it should be explained in more detail that since the photographic optical system takes the form of a convex lens as a whole, it is better, in view of the aberrational problem and the like, that the front or first lens surface have a convex curvature toward the front. For this reason, most of the ordinary photographic optical systems have their frontmost lens surfaces figured shaped from convex to flat. Particularly for cameras usable under water, it is common practice to make use of a flat glass plate as the protection member for the optical system. In this case, as shown in FIGS. 1(A) and 1(B) for in-the-air and underwater photography respectively, the object point which is sharply focused lies at a farther distance under water than in the air. Now letting l and l' denote the distance from a protection glass plate 1 to object points 4 and 4' of which images are sharply focused on a film plane 3 by the photographic lens 2, and n.sub.w is the index of refraction of water, we have EQU l'=n.sub.w l
For the given value of the distance l=3 meters, as the refractive index of water n.sub.w =1.33, l'=4 meters is obtained. This is also valid when the protection glass plate 1 is not used, and, instead, the front member of the optical system having a convex front surface toward the front constitutes part of the underwater housing.
In underwater photography, however, fish, coral and the like are principal objects intended to be photographed and are generally shot close up. Another fact is that as the water object distances becomes longer than 5 to 6 meters, the water turbidity of water rapidly increases so that only unclear photographs will be taken. That is, since it is under water that close photography prevails the shift of focal point the photographic optical system focal point for an object under water to a distance greater than in the air is very disadvantageous.
An object of the invention is to provide an amphibious camera which has, despite the fixed in-focus position and in view of quick and easy under water management, to be focused on an object at a substantially shorter distance when under water than when in the air, while still permitting good imaging performance not only in the air but also under water. The feature resides in that as applied to a camera which has a housing constructed for in-the-air photography in the watertight form, the optical system comprises, a front lens component and a rear lens component which satisfies the following conditions: EQU (a) -90fII&lt;R1&lt;-2.5fII EQU (b) 0&lt;R1/R2&lt;23.4
where R1 and R2 are the radii of curvature of the front and rear lens surfaces of the front component respectively, and fII is the focal length of a combination of the rear lens surface of the front component with the rear component.